The measurement of moisture content is important in many industrial processes. The conventional methods of moisture measurement, based on oven-drying of a sample taken from the process stream, are frequently not rapid enough for quality control purposes and rely on the small sample analysed being representative of the total stream. Consequently, there is a requirement for on-line analysis techniques that provide a continuous analysis of the process stream.
One important example is in the on-line measurement of moisture in coal carried on a conveyor belt. In recent years there has been a growing interest in measuring and controlling the moisture content of coking and steaming coals. Moisture in these coals has increased mainly because of the increased proportion of fine coal in preparation plants as a result of modern mining methods and because greater emphasis is placed on fine coal recovery for economic reasons. Lower product coal moisture results in lower transport costs, energy savings when the coal is utilised and reduced handling difficulties.
A number of techniques have been investigated for the on-line determination of moisture in coal. These include capacitance measurement, microwave transmission, fast neutron and gamma-ray transmission, infrared reflectance and nuclear magnetic resonance.
Moisture measurement by microwave techniques is based either on the high real or imaginary part of the complex dielectric constant of water compared to the dielectric properties of the majority of dry materials. The complex dielectric constant (.epsilon. = .epsilon.' - j .epsilon.") is a function of the dielectric constant (.epsilon.') and the loss factor (.epsilon.") of a material, and for coal is not only influenced by the moisture content, but also other material properties such as bulk density, ash content and coal rank. The parameter most frequently used is a measurement of the attenuation or power loss of a transmitted beam of microwaves at a frequency in the range of 2-10 GHz, and for on-belt measurement of moisture in coal, a measurement of bulk density and/or coal layer thickness is also required.
A measurement of microwave attenuation has been widely used for the determination of moisture content in a range of materials. However, the attenuation measurement is very dependent on the sample thickness and sample position relative to the microwave source, and therefore use of the technique in on-line measurement applications has relied on presentation of a constant thickness layer of material to the gauge. This has been achieved using devices such as a levelling plough or a recirculating levelling conveyor that level the top surface of material carried on a conveyor belt. A disadvantage of these techniques is that contacting devices of this type may cause blockages in the flow of material. An alternative technique is to perform measurements on a sample by-line where sample presentation can be carefully controlled, however, this is more costly and the frequency of the analysis is determined by the rate of sampling.
A number of attempts have been made to modify the attenuation measurement technique to improve its accuracy for the determination of moisture content. These have involved an improved design of microwave apparatus to minimise interferences from reflection at the antennae, the use of reference signals to overcome problems with equipment drifts and the use of lens corrected antennae to improve signal directivity. However, the predominant error in the measurement of attenuation results from amplitude modulation of the transmitted signal by signals reflected from the sample/air interfaces. One proposed method of reducing the error resulting from such amplitude modulation is to measure the attenuation at 50-100 frequencies in a 0.5-1 octave frequency band and correlate the mean value with moisture content. This technique is reported to reduce the influence of variation in sample thickness and position on the attenuation measurement.
A measurement of microwave phase shift has also been investigated for the determination of moisture. In the on-line measurement of moisture by this technique the measured phase shift or attenuation are normalized to the sample mass per unit area determined from a gamma-ray transmission measurement.
A combined measurement of phase shift and attenuation has also been proposed to determine moisture independent of variations in the sample density. In this technique the moisture content is determined from the difference in the attenuation at two measurement frequencies and a measurement of phase shift compensates for density variation. However, for on-line measurement either a constant thickness sample or a measurement of sample thickness is required.